This invention relates to a process for sintering or reaction sintering ceramic or refractory materials using plasma heated gases.
In practice, many ceramic or refractory materials, such as those consisting of alumina and silica, are sintered in tunnel or periodic kilns which are fired by energy released from the combustion of fossil fuels with air or oxygen. If the ceramic or refractory material can be exposed to air and/or the products of combustion, then the kiln may be directly fired, in which case, the heating and utilization of energy may be reasonably efficient. However, for certain ceramic materials, such as the carbides, nitrides and borides, the firing must be done in the absence of oxygen or oxygen-bearing gases, including water and carbon dioxide, to prevent formation of oxides, which may have undesirable physical or chemical properties. Under such conditions, fossil fuel-fired furnaces may be used but the ceramic or refractory materials must be kept in a controlled environment isolated from the combustion products of the fuel. Because the ceramic or refractory materials must be heated in a retort, the heating is indirect, inefficient and slow. On a commercial scale such a process, using a tunnel kiln, for example, requires about 70-90 hours (including the cooling cycle).
Electric kilns are also used to sinter ceramic or refractory materials under controlled atmospheres, but also tend to be energy inefficient and slow. In the case of a kiln equipped with heating elements such as graphite electrodes, the voltage can be controlled and the kiln can be heated to fairly high temperatures, yet there are several disadvantages: (1) The heating elements have a limited size, complex shape and must be kept under a strictly controlled atmosphere to maintain a long life; and, (2) Furnace size is limited and it is difficult to achieve a uniform temperature in this type of kiln because only the heating elements are the source of the radiant heat. Because of this radiant heat transfer, as well as a size limit for heating elements, the kiln has a limited productivity and a poor energy efficiency.
Conventional processes, in general, require long retention times resulting in poor energy utilization, excessive furnace gas consumption and high maintenance costs.
In the sintering or reaction sintering of ceramic or refractory materials, the required reaction temperature of the furnace is usually less than 2500.degree. C. However, the average temperature of gases heated through a plasma arc column is above 7000.degree. C.; thus, plasma technology has previously been applied only to fusion of high temperature materials and not to sintering or reaction sintering. In order to use plasma heated gases at a lower furnace temperature, they can be mixed with secondary gases. Prior art plasma systems for mixing plasma heated gases and secondary gases generally employ radial injection of the secondary gases into the plasma gas and single rather than multiple plasma torches. In a furnace disclosed in U.S. Pat. No. 3,935,371 by Camacho et al. entitled "Plasma Heated Batch-Type Annealing Furnace", the plasma torch and reactant gas inlet are positioned on the bottom of the furnace and flow in the same direction.